CLI: let --export-settings - write the merged config JSON to stdout (#15698)

`--export-settings` already writes the merged config as JSON at the
right point in the CLI flow. Passing `-` now writes that same document
to stdout, so scripts can inspect the effective config without a temp
file. This replaces #14605.

- `ConfigBase::save_to_json` gains a stream overload. The file overload
serializes through it before opening the file, so the output format is
unchanged, and a config that cannot be serialized (invalid UTF-8) now
leaves the existing file untouched instead of truncating it.
- On stdout, invalid UTF-8 in string values is written as U+FFFD instead
of ending the process with an uncaught `type_error`. Files keep the
strict behaviour.
- To keep stdout pure JSON, `-` is rejected up front (stderr message,
`CLI_INVALID_PARAMS`, shell status 254) when combined with an action or
transform that can write to stdout or does real work: `--info`,
`--help`, `--orient`, slicing and exporting. Options that do nothing
without a slice (`--uptodate`, `--min-save`, `--pipe`, ...) are still
accepted.
- The one unconditional stdout write on a success path, "skip locked
instance" during arrange, now goes to the log.
- Every other value, including the default `output.json`, behaves as
before.

Tests in `tests/libslic3r/test_config.cpp`: the stream output equals the
file output and keeps the tab-indented format; invalid UTF-8 throws on
the strict path and is replaced when asked; a failed save leaves the
previous file intact.

src/libslic3r/Config.cpp

@@ -7,6 +7,7 @@
 #include <algorithm>
 #include <assert.h>
 #include <fstream>
+#include <sstream>
 #include <iostream>
 #include <iomanip>
 #include <regex>
@@ -1515,6 +1516,19 @@ std::optional<PluginCapabilityRef> parse_capability_ref(const std::string& value
 
 //BBS: add json support
 void ConfigBase::save_to_json(const std::string &file, const std::string &name, const std::string &from, const std::string &version) const
+{
+    // Serialize first: if that throws (invalid UTF-8), the existing file stays untouched.
+    std::ostringstream ss;
+    this->save_to_json(ss, name, from, version);
+    boost::nowide::ofstream c;
+    c.open(file, std::ios::out | std::ios::trunc);
+    c << ss.str();
+    c.close();
+
+    BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ":" <<__LINE__ << boost::format(", saved config to %1%\n")%file;
+}
+
+void ConfigBase::save_to_json(std::ostream &os, const std::string &name, const std::string &from, const std::string &version, bool replace_invalid_utf8) const
 {
     json j;
     //record the headers
@@ -1561,12 +1575,7 @@ void ConfigBase::save_to_json(const std::string &file, const std::string &name,
             j["plugins"] = unique_refs;
     }
 
-    boost::nowide::ofstream c;
-    c.open(file, std::ios::out | std::ios::trunc);
-    c << j.dump(1, '\t') << std::endl;
-    c.close();
-
-    BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ":" <<__LINE__ << boost::format(", saved config to %1%\n")%file;
+    os << j.dump(1, '\t', false, replace_invalid_utf8 ? json::error_handler_t::replace : json::error_handler_t::strict) << std::endl;
 }
 
 void ConfigBase::save(const std::string &file) const

src/libslic3r/Config.hpp

@@ -2825,6 +2825,9 @@ public:
 
     //BBS: add json support
     void save_to_json(const std::string &file, const std::string &name, const std::string &from, const std::string &version) const;
+    // Same document, written to a stream. Invalid UTF-8 in a string value throws nlohmann's type_error unless
+    // replace_invalid_utf8 is set, which writes U+FFFD instead (for callers such as stdout with no handler).
+    void save_to_json(std::ostream &os, const std::string &name, const std::string &from, const std::string &version, bool replace_invalid_utf8 = false) const;
 
     // Rebuild the in-memory "plugins" manifest (the "name;uuid;capability" references the plugin
     // dispatchers consume) from the plugin-backed options via the registered resolver. save_to_json()

src/libslic3r/PrintConfig.cpp

@@ -11916,7 +11916,7 @@ CLIActionsConfigDef::CLIActionsConfigDef()
 
     def = this->add("export_settings", coString);
     def->label = L("Export Settings");
-    def->tooltip = L("This exports settings to a file.");
+    def->tooltip = L("This exports settings to a file. Use - to write them to stdout.");
     def->cli_params = "settings.json";
     def->set_default_value(new ConfigOptionString("output.json"));